Mimicking neural coding in retinal ganglion cells with short pulse electrical stimulation

ABSTRACT

A method, device and system for stimulating visual tissue, typically in the retina or visual cortex, to achieve an artificial percept of light or image. The method includes providing stimulating electrodes suitable for placement in proximity to the visual tissue and generating a series of short-duration stimulation signals having a duration of less than about 0.5 milliseconds each. The short-duration stimulation signals are applied through the stimulating electrodes with varying frequencies that are substantially matched to a spiking range of frequencies of at least one ganglion cell for perceiving brightness or image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional application of U.S. patentapplication Ser. No. 11/293,331, filed Dec. 1, 2005 now U.S. Pat. No.8,103,352 which claims benefit of the U.S. Provisional PatentApplication Nos. 60/675,981 filed on Apr. 28, 2005, entitled MimickingNeural Coding in Retinal Ganglion Cells with Short Pulse ElectricalStimulation; 60/661,283 filed on Mar. 11, 2005, entitled A StimulusParadigm for Precise Temporal Control of Retinal Spiking Elicited byProsthetic Devices; and 60/632,929 filed on Dec. 3, 2004, entitledMimicking Neural Coding in Retinal Ganglion Cells with Short PulseElectrical Stimulation. The provisional patent applications areincorporated herein by reference, in their entirety, for all purposes.

GOVERNMENT RIGHTS NOTICE

This invention was made with government support under grant No.R24EY12893-01, awarded by the National Institutes of Health. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is generally directed to neural stimulation andmore specifically to control of repetitive neural spikes and further tocontrol perceived brightness by mimicking neural coding in a visualprosthesis.

BACKGROUND OF THE INVENTION

In 1755 LeRoy passed the discharge of a Leyden jar through the orbit ofa man who was blind from cataract and the patient saw “flames passingrapidly downwards.” Ever since, there has been a fascination withelectrically elicited visual perception. The general concept ofelectrical stimulation of retinal cells to produce these flashes oflight or phosphenes has been known for quite some time. Based on thesegeneral principles, some early attempts at devising a prosthesis foraiding the visually impaired have included attaching electrodes to thehead or eyelids of patients. While some of these early attempts met withlimited success, these early prosthetic devices were large, bulky andcould not produce adequate simulated vision to truly aid the visuallyimpaired.

In the early 1930's, Foerster investigated the effect of electricallystimulating the exposed occipital pole of one cerebral hemisphere. Hefound that when a point at the extreme occipital pole was stimulated,the patient perceived a small spot of light directly in front andmotionless (a phosphene). Subsequently, Brindley and Lewin (1968)thoroughly studied electrical stimulation of the human occipital(visual) cortex. By varying the stimulation parameters, theseinvestigators described in detail the location of the phosphenesproduced relative to the specific region of the occipital cortexstimulated. These experiments demonstrated: (1) the consistent shape andposition of phosphenes; (2) that increased stimulation pulse durationmade phosphenes brighter; and (3) that there was no detectableinteraction between neighboring electrodes which were as close as 2.4 mmapart.

As intraocular surgical techniques have advanced, it has become possibleto apply stimulation to small groups of individual retinal cells togenerate focused phosphenes through devices implanted within the eyeitself. This has sparked renewed interest in developing methods andapparati to aid the visually impaired. Specifically, great effort hasbeen expended in the area of intraocular retinal prosthesis devices inan effort to restore vision in cases where blindness is caused byphotoreceptor degenerative retinal diseases such as retinitis pigmentosaand age related macular degeneration which affect millions of peopleworldwide.

Neural tissue can be artificially stimulated and activated by prostheticdevices that pass pulses of electrical current through electrodes onsuch a device. The passage of current causes changes in electricalpotentials across visual neuronal membranes, which can initiate visualneuron action potentials, which are the means of information transfer inthe nervous system.

Based on this mechanism, it is possible to input information into thenervous system by coding the information as a sequence of electricalpulses which are relayed to the nervous system via the prostheticdevice. In this way, it is possible to provide artificial sensationsincluding vision.

One application of neural tissue stimulation is in the rehabilitation ofthe blind. Some forms of blindness involve selective loss of the lightsensitive transducers of the retina. Other retinal neurons remainviable, however, and may be activated in the manner described above byplacement of a prosthetic electrode device on the inner (toward thevitreous) retinal surface (epiretinal). This placement must bemechanically stable, minimize the distance between the device electrodesand the visual neurons, and avoid undue compression of the visualneurons.

In 1986, Bullara (U.S. Pat. No. 4,573,481) patented an electrodeassembly for surgical implantation on a nerve. The matrix was siliconewith embedded iridium electrodes. The assembly fit around a nerve tostimulate it.

Dawson and Radtke stimulated cat's retina by direct electricalstimulation of the retinal ganglion cell layer. These experimentersplaced nine and then fourteen electrodes upon the inner retinal layer(i.e., primarily the ganglion cell layer) of two cats. Their experimentssuggested that electrical stimulation of the retina with 30 to 100 .mu.Acurrent resulted in visual cortical responses. These experiments werecarried out with needle-shaped electrodes that penetrated the surface ofthe retina (see also U.S. Pat. No. 4,628,933 to Michelson).

The Michelson '933 apparatus includes an array of photosensitive deviceson its surface that are connected to a plurality of electrodespositioned on the opposite surface of the device to stimulate theretina. These electrodes are disposed to form an array similar to a “bedof nails” having conductors which impinge directly on the retina tostimulate the retinal cells. U.S. Pat. No. 4,837,049 to Byers describesspike electrodes for neural stimulation. Each spike electrode piercesneural tissue for better electrical contact. U.S. Pat. No. 5,215,088 toNormann describes an array of spike electrodes for cortical stimulation.Each spike pierces cortical tissue for better electrical contact.

The art of implanting an intraocular prosthetic device to electricallystimulate the retina was advanced with the introduction of retinal tacksin retinal surgery. De Juan, et al. at Duke University Eye Centerinserted retinal tacks into retinas in an effort to reattach retinasthat had detached from the underlying choroid, which is the source ofblood supply for the outer retina and thus the photoreceptors. See,e.g., E. de Juan, et al., 99 Am. J. Ophthalmol. 272 (1985). Theseretinal tacks have proved to be biocompatible and remain embedded in theretina, and choroid/sclera, effectively pinning the retina against thechoroid and the posterior aspects of the globe. Retinal tacks are oneway to attach a retinal array to the retina. U.S. Pat. No. 5,109,844 tode Juan describes a flat electrode array placed against the retina forvisual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes aretinal prosthesis for use with the flat retinal array described in deJuan.

Retinal prosthetics use electricity to stimulate nerve cells. To beeffective, the pattern of electrical stimulation should produce signalsin the ganglion cells that mimic the signals they would receive undernormal conditions. A major problem in mimicking normal neural activityis how to encode light intensity or brightness. Spiking neurons encodeinformation in trains of action potentials, and it is generally acceptedthat most of the information about the intensity/brightness of thestimulus is encoded in the rate of action potentials provided to thebrain through the optical nerve. Thus, there is a need to provide amethod and apparatus to preferentially directly stimulate the ganglioncells with rate-coded intensity information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a perspective view of an implantableportion of a visual prosthesis in accordance with an embodiment of thepresent invention.

FIG. 2 is an illustration of a perspective side view of an implantableportion of the visual prosthesis showing a fan tail in more detail inaccordance with an embodiment of the present invention.

FIG. 3 is an illustration of an perspective edge view of the implantableportion of the visual prosthesis showing a hook for aiding theimplantation of the retinal prosthesis.

FIG. 4 is an illustration of an external profile view of a user wearingan external portion of a visual prosthesis system.

FIG. 5 is an illustration of a schematic representation of a region ofretinal tissue being stimulated by an electrode, which is stimulating atleast one ganglion cell.

FIG. 6A is an illustration of a typical spiking response to a long-pulseelectrical stimulation FIG. 6B is an illustration of a ganglion cellspike response generated by the long-pulse electrical stimulation shownin FIG. 6A, after subtracting the stimulus artifact.

FIG. 7A is an illustration of a ganglion cell response when stimulatedby a short-pulse stimulation.

FIG. 7B is an illustration of a spike response elicited from a ganglioncell when stimulated by a short-pulse electrical stimulation.

FIG. 8 is an illustration of how the longer-duration stimulation pulsesproduce early and late phase responses in the ganglion cells.

FIG. 9A is an illustration of a light source producing light-elicitedspike response pattern.

FIG. 9B is an illustration of a short-duration stimulation pulse patternreplicating/mimicking the light-elicited spike response of FIG. 9A.

FIG. 10 is an illustration of the spiking responses elicited in aganglion cell to a short-duration stimulation pulse having varyingcurrent amplitudes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

FIG. 1 is an illustration of a perspective view of an implantableportion of a visual prosthesis in accordance with an embodiment of thepresent invention. An electrode array 10 is mountable by a retinal tackor similar means to the epiretinal surface. The electrode array 10 iselectrically coupled by a cable 12 which pierces the sclera and iselectrically coupled to an electronics package 14, external to thesclera.

The electronics package 14 is electrically coupled to an inductive coil16. Preferably the inductive coil 16 is made from wound wire.Alternatively, the inductive coil may be made from a thin film polymersandwich with wire traces deposited between layers of thin film polymer.The electronics package 14 and inductive coil 16 are held together by amolded body 18. The molded body 18 may also include suture tabs 20. Themolded body narrows to form a strap 22 which surrounds the sclera andholds the molded body 18, inductive coil 16, and electronics package 14in place. The molded body 18, suture tabs 20 and strap 22 are preferablyan integrated unit made of silicone elastomer. Silicone elastomer can beformed in a pre-curved shape to match the curvature of a typical sclera.However, silicone remains flexible enough to accommodate implantationand to adapt to variations in the curvature of an individual sclera. Theinductive coil 16 and molded body 18 are preferably oval shaped. A strapcan better support an oval shaped coil.

It should be noted that the entire visual prosthesis is attached to andsupported by the sclera. Even though eye motion is useless in the blind,it often continues long after a person has lost their sight. In theembodiment of the present invention it is provided for the entireimplantable portion of the prosthesis to be attached to and supported bythe sclera. By placing the visual prosthesis under the rectus muscleswith the electronics package in an area of fatty tissue between therectus muscles, eye motion does not cause any flexing which mightfatigue, and eventually damage, the visual prosthesis.

FIG. 2 is an illustration of a side view of an implantable portion ofthe visual prosthesis showing a fan tail in more detail in accordancewith an embodiment of the present invention. When implanting the visualprosthesis, the strap 22 should pass under the eye muscles to surroundthe sclera. The inductive coil 16 and molded body 18 should also followthe strap under the lateral rectus muscle on the side of the sclera. Theimplantable portion of the visual prosthesis is very delicate. It iseasy to tear the molded body 18 or break wires in the inductive coil 16.In order to allow the molded body 18 to slide smoothly under the lateralrectus muscle, the molded body is shaped in the form of a fan tail 24 onthe end opposite the electronics package 14.

Referring to FIG. 3, reinforced attachment points 26 are provided tofacilitate handling of the visual prosthesis by surgical tools.Preferably, the reinforced attachment points are harder silicone formedaround holes through the molded body 18. Further, a hook 28 is moldedinto the strap 22 just beyond the end of the fan tail 24. A surgicaltool can be used against the hook 28 to push the strap 22 under therectus muscles. The hook 28 is more clearly depicted by the edge view ofFIG. 3. The strap 22 is attached to itself by a sleeve 23. The sleeve 23is a friction device that connects two silicone bands and holds themtogether with friction. The sleeve 23 is similar to a Watzke sleeve,used with a scleral buckle, and is well known in the art.

In the preferred embodiment, the electrode array 10 and cable 12 areformed layers of a thin polymer film with metal traces sandwichedbetween the thin polymer films. In such an embodiment, it isadvantageous that the film with openings for electrode array 10 be thesame film with an opening for connection to the electronics package 14.Therefore, the cable 12 exits the electronics package up away from thefantail 24, folds over itself and exits down toward the fantail 24,before turning at a right angle and piercing the sclera. This allows thesame side of the cable to face both the electronics package and theretina. The cable 12 may also include a fantail at the point it isattached to the electronics package 14 and at the point it is attachedto the electrode array 10 to reduce any stress on the connections thatmay be caused by implantation. The cable should exit the molded body 18toward the front of the eye. The cable should travel above the lateralrectus muscle and pierce the sclera at the pars plana, in front of theretina, so it does not damage the retina. Once inside the eye, the cable12 can fold back over the retina to properly locate the electrode array10 on the epiretinal surface.

FIG. 4 is an illustration of an external profile view of a user wearingan external portion of a visual prosthesis system. The external portionmay be built into the temple of a pair of glasses. An image processingunit in a form of a camera 30 collects a video image and transmits datato an external electronics package 32. The image processing unit isadapted for detecting features of surrounding objects and generatingimage signals based on the detected features of the surrounding objects.A battery 34 powers the camera 30, external electronics package 32, andprovides power to a primary inductive coil 36. The primary inductivecoil 36 sends power and data through the skin and skull to the inductivecoil 16. Maximum efficiency is obtained when the primary inductive coil36 and inductive coil 16 are the same size, shape and as close togetheras possible.

FIG. 5 is an illustration of a schematic representation of a region ofretinal tissue being stimulated by an electrode 40, which is stimulatingat least one ganglion cell 42, bypassing the photoreceptor cells 46 andbipolar cells 44. In order to restore meaningful vision to blindpatients, individual prosthetic electrodes of a visual prosthetic deviceand system, described above in connection with FIGS. 1-4, should elicitspecific spiking patterns in ganglion cells. These patterns shouldclosely resemble the normal light-elicited patterns. In the embodimentsof the present invention, it is proposed that a method/stimulus protocolis developed that reliably elicits one ganglion cell spike for eachelectrical pulse i.e., response for each electrical stimulation impartedon each ganglion cell. The ability to reliably generate individualspikes with high temporal precision allows us to replicate a wide rangeof spike patterns. Although, the electrical stimulation signals may bedifferent types of signals/waveforms, herein it is contemplated that thestimulation signals are in the form of stimulation pulses.

Broadly, in the embodiments of the present invention, a method, a visualprosthesis and a visual prosthesis system is proposed for stimulating alocalized portion of visual tissue. The visual tissue may be at leastone visual neuron wherein the visual neuron may be more specifically aretinal neuron. Yet more specifically, as described above, the retinalneuron may be at least one ganglion cell. The method comprises the stepsof: a) providing at least one stimulating electrode suitable forplacement in proximity to the visual tissue; b) generating at least onestimulation signal, said at least one stimulation signal having aduration of less than about 0.5 milliseconds; and c) applying the atleast one stimulation signal to the at least one stimulating electrode,thereby stimulating the localized portion of the visual tissue.

In the experiments conducted, patch clamp recordings were used tomeasure spiking responses from individual retinal ganglion cells in theflat mounted rabbit retina. Small tipped platinum-iridium epiretinalelectrodes were used to deliver biphasic electrical stimulus pulses withstimulation frequencies that ranged from 1-250 Hz. With regard to thestimulation pulses, it is proposed to deliver balanced biphasic currentpulses to patients to reduce the biologically harmful product ofelectrochemical reactions. The pulses are delivered with thecathodic/negative pulse first and then followed by the anodic/positivepulse in order to equalize the cellular charge delivered. In thismanner, the biphasic pulse delivers approximately zero net charge. It isproposed that other types of unbalanced pulses may be utilized thatwould not have harmful biological effects. Moreover, in the presentembodiment, it is proposed to stimulate at least one ganglion cell at afrequency greater than about 10 Hz.

To distinguish spiking elicited by direct activation of the ganglioncell from spiking elicited by activation of presynaptic cells, synapticinputs to ganglion cells were blocked pharmacologically. Light responsesand dendritic morphology were used to identify each ganglion cell type.Referring to FIG. 5, the presynaptic cells also referred to as thedeeper retinal cells are generally the bipolar cells 44. In a peopleblinded by retinitis pigmentosa or macular degeneration, thephotoreceptors 46 and the bipolar cells 44 may be damaged. Therefore, itis desired to electrically stimulate the ganglion cells 42 directly toachieve a better visual response.

It was realized that long duration electrical pulses of greater thanabout 1 msec. elicited a single spike within 0.5 msec. of the pulseonset followed by a train of spikes which could persist for more than 50msec. depending on pulse amplitude levels. Pharmacological blockers ofexcitatory synaptic input eliminated all but the first spike suggestingthat the first spike arises from direct activation of the ganglion celland all other spikes arise from depolarization due to excitatory inputfrom presynaptic cells.

Experimental evidence suggests that short biphasic pulses preferentiallystimulate ganglion cells to the substantial exclusion of directstimulation of the deeper retinal cells. These short pulses each producea single spike in ganglion cells within 1 msec. As described, in anembodiment of the present invention, it is proposed to apply at leastone stimulation signal, for example, in the form of a biphasic pulse, toa localized portion of visual tissue, wherein the stimulation signal hasa duration of less than about 0.5 msec. In further aspects of theembodiment of the present invention, it is proposed to apply stimulationsignals having durations of less than about 0.3 msec. preferably lessthan 0.1 msec. It is further proposed that each stimulation signal, inthe form of a stimulation pulse, should have a predeterminedsubstantially constant amplitude.

Short duration pulses elicit one spike per pulse at all stimulationfrequencies; therefore they can be used to generate precise temporalpatterns of activity in ganglion cells. These patterns can be used tomimic physiologically relevant light evoked responses, e.g., they canreplicate the spiking pattern of transient or sustained cells, and alsomimic the changes in spike frequency that underlie responses to changinglight intensities and contrasts. Furthermore, it is proposed that inaddition to light intensities and contrasts, various images may also bemimicked as different spiking patterns of varying spike frequency suchthat images may be perceived by the patient. This may be accomplished bystimulating at least one ganglion cell with stimulation signals/pulseshaving varying frequencies corresponding to light-evoked frequencypatterns associated with respective images.

Conventionally, the visual perception of low intensity light has beenachieved by applying low amplitude pulses, whereas the perception ofhigh intensity light has been achieved by applying high amplitude pulsesto the retinal tissue. In contrast, in the embodiments of the presentinvention, it is proposed that in order to encode theintensity/brightness of an external stimulus, for example, an image, therate of the pulses provided by the electrodes to the ganglion cells maybe varied such that low intensities are represented with low stimulationpulse frequencies and high intensities are represented with high pulsefrequencies. The lower and upper bounds of pulse frequency and therelationship between brightness and spike rate should be matched to thenormal spiking range of retinal ganglion cells. In other words, iscontemplated to stimulate at least one ganglion cell with stimulationsignals having varying frequencies corresponding to varying levels ofbrightness such that the varying frequencies are substantially matchedto a spiking range of frequencies of the at least one ganglion cell forperceiving brightness.

In another embodiment of the present invention, it is proposed tostimulate a localized portion of visual tissue, more particularly atleast one ganglion cell, at a frequency greater than about 10 Hznotwithstanding the duration of the stimulation signals. It iscontemplated that ganglion cell-stimulation at frequencies greater thanabout 10 Hz elicits one spike per stimulation pulse.

In the experiments conducted, measurements were made of the spikingresponses in ganglion cells, the output cells of the retina, usingcell-attached patch clamp recordings in the flat mount rabbit retina.Electrical stimulation was delivered by small tipped platinum-iridium(Pt—Ir) electrodes (Impedance: 10-100 k.OMEGA.) and consisted ofbiphasic (cathodic first), charge balanced square wave current pulses.Amplitudes ranged from 1 to 400 .mu.A and pulse durations ranged from 60.mu.sec to 6 msec. Light responses were measured continuously duringexperiments to ensure stability of the recording setup.

FIG. 6A is an illustration of a typical spiking response to a long-pulseelectrical stimulation. Large transient currents in the form of sharplarge signals 50 were recorded which temporally correlate with the onset52 and terminations 54 of the individual phases of the stimulus pulse56. These are electrical ‘artifacts’ that arise from the application ofstimulus pulse and tend to obscure a neural action potential (spike)buried in the trace. By applying tetrodotoxin (TTX), a blocker ofneuronal spiking, the response was different but only in the regionimmediately following the onset of the pulse. Subtracting the TTXresponse 58 from the control response 60 unmasked a single spike 62 (seeFIG. 6B) which was similar in magnitude and kinetics to light evokedspikes. In response to the anodic phase of the stimulus pulse, there wasno difference between the control and TTX responses.

FIG. 6B is an illustration of a ganglion cell spike response generatedby the long-pulse electrical stimulation shown in FIG. 6A, aftersubtracting the stimulus artifact recorded during application of TTX 58.FIG. 6A. The spike 62 was elicited immediately after the onset 52 of thepulse 56 as shown in FIG. 6B, suggesting that it was elicited at theleading edge of the pulse 56. In order to further test this observation,the duration of the stimulus pulse was shortened.

FIG. 7A is an illustration of a ganglion cell response when stimulatedby a short-stimulus-pulse. FIG. 7A shows the response to a short (0.2ms) pulse 64 under normal conditions 66 and during the application ofTTX 68. As in FIG. 6, subtracting the TTX response from the normalresponse to eliminate the stimulus artifact reveals the presence of asingle spike 70 (see FIG. 7B).

FIG. 7B is an illustration of a single spike response elicited from aganglion cell when stimulated by a single short-pulse electricalstimulation after subtraction of the electrical stimulation pulseartifact. The response to a short pulse under normal conditions wasalways triphasic consisting of a large upward and downward deflectionfollowed by a smaller upward deflection. In TTX the response was alwaysbiphasic 68 (no second upward deflection). Since TTX eliminates allspiking activity in ganglion cells, the small upward deflection that iseliminated by TTX represents a spiking response from the ganglion cell.In later experiments with short duration pulses the second upwarddeflection was a reliable marker to indicate that spiking had beenelicited.

Longer duration stimulation pulses elicited spiking that occurred aftercompletion of the stimulus pulse, in addition to the spike produced bythe leading edge of the stimulus. FIG. 8 illustrates that thelonger-duration stimulation pulse produce early and late phase spikes inthe ganglion cell. As shown in FIG. 8, the longer-duration stimulationpulse 72 results in an early phase spike response 74 and a late phasespike responses 76. The number and timing of the late phase spikeresponses was variable and generally increased with increasingstimulation pulse amplitude or duration. Short duration stimulationpulses less than about 0.15 ms did not elicit any of these late phasespike responses. All of the late phase spiking responses were eliminatedin the presence of a cocktail of drugs that blocked excitatory synapticinput to ganglion cells, while leaving the initial short latency spikeunaffected. This indicates that long-duration stimulation pulsesactivate presynaptic neurons that release excitatory neurotransmitterwhich in turn stimulates ganglion cells to produce additional spikes.The early phase single spike response was not eliminated by drugs thatblock synaptic input from presynaptic cells, indicating that it arisesfrom direct activation of the ganglion cell.

At higher stimulation frequencies namely, greater than about 10 Hz,short duration stimulation pulses continued to elicit one spike responseper stimulation pulse. This was consistent for stimulation frequenciesup to 250 Hz. To test if light-elicited spike response patterns could bereplicate with short-pulse electrical stimulation, an experiment wasconducted. FIG. 9A is an illustration of the spike response pattern 82produced by a 1 second long pulse of 100 .mu.m square light stimulus. Atrain of short electrical pulses was then created that had the sametemporal pattern as the measured light-evoked spike train 84. FIG. 9Bshows that when this pulse pattern was used to stimulate the retina, theresulting spiking pattern 86 accurately replicated/mimicked the patternproduced by the light stimulus.

A single spike per pulse could be reliably elicited over a wide range ofpulse amplitudes. FIG. 10 is an illustration of the spiking responseselicited in a ganglion cell to a short-duration stimulation pulse havingvarying current amplitudes. As shown in FIG. 10, pulse amplitudes above120 .mu.A consistently elicited a single spike response whereas pulseamplitudes above 360 .mu.A elicited late phase spikes which are likelydue to activation of presynaptic neurons in addition to the early phasespike.

A stimulus paradigm is developed that elicits one neuronal spike perstimulus pulse and is effective over a wide range of stimulationfrequencies and amplitudes. This allows electrical prosthetic devices tomimic the spike patterns sent to the brain during normal vision.

Short pulses are likely to simplify the generation of spatially complexpatterns of activity because they activate only ganglion cells. Longerpulses appear to activate bipolar cells. Amacrine cells are alsoactivated, either directly or via synaptic input from bipolar cells.Activation of these presynaptic elements elicits long latency excitationas well as long latency inhibition that can spread over broad spatialregions. The use of short pulses avoids the long latency and broadspatial activity.

Short pulses elicit spiking in ganglion cells using less total chargethan long pulses. This may allow for reduced electrode size therebygenerating a more focal response.

In addition to encoding intensity information, the embodiments of thepresent invention could be used to encode other visual attributes withelectrical stimulation. Visual information that reaches the brain can berelayed as a pattern of action potentials transmitted down the fibers ofthe optic nerve. Cells that provide information about specific stimulusattributes, such as color, generally tile the spatial surface of theretina. These cells provide information about the magnitude of any ofthe attribute that they encode through variations in the rate and timingof their action potentials or spikes. Therefore, the embodimentdescribed here for generating precise patterns of spiking could be usedto electrically generate a given stimulus attribute, such as color, byselectively stimulating a particular cell with a predeterminedelectrical pulse sequence designed to mimic the cell's normal responseto that stimulus attribute.

While the invention has been described by means of specific embodiments,it is understood that numerous modifications and variations could bemade thereto by those skilled in the art without departing from thespirit and scope of the invention. In particular, one of skill in theart would realize that, while described in terms of retinal stimulation,the principles disclosed are equally applicable to cortical visualstimulation, as well as many other forms of neural stimulation. It istherefore to be understood that within the scope of the claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A method of stimulating a localized portion of visual tissuecomprising the steps of: providing a two dimensional array ofstimulating electrodes suitable for placement in proximity to the visualtissue; generating a plurality of stimulation signals adapted to mimicneural coding by eliciting a single spike response in at least onevisual neuron per stimulation pulse, wherein said generating comprisesselecting an amplitude and selecting a frequency of greater than orequal to 1 Hertz and less than or equal to 250 Hertz; and applying saidplurality of stimulation signals to said two dimensional array ofstimulating electrodes, thereby stimulating the localized portion of thevisual tissue and creating perception of light in formed images.
 2. Themethod of claim 1, wherein said plurality of stimulation signalscomprise biphasic signals having a negative portion and a positiveportion.
 3. The method of claim 2, wherein the localized portion of thevisual tissue comprises at least one visual neuron.
 4. The method ofclaim 3 wherein the at least one visual neuron is at least one retinalneuron.
 5. The method of claim 4, wherein the at least one retinalneuron is at least one ganglion cell.
 6. The method of claim 3, whereinsaid stimulating the localized portion comprises: stimulating at leastone ganglion cell to the substantial exclusion of direct stimulation ofthe deeper retinal cells.
 7. The method of claim 6, further comprising;stimulating the at least one ganglion cell with stimulation signalshaving varying frequencies corresponding to varying levels ofbrightness.
 8. The method of claim 1, wherein said generating furthercomprises selecting a duration of less than about 1 millisecond for eachof the stimulation signals.